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[samba.git] / docs / docbook / projdoc / NT_Security.xml

<chapter id="unix-permissions">
<chapterinfo>
        &author.jeremy;
        <pubdate>12 Apr 1999</pubdate>
</chapterinfo>

<title>UNIX Permission Bits and Windows NT Access Control Lists</title>

<sect1>
        <title>Viewing and changing UNIX permissions using the NT 
        security dialogs</title>

        <para>Windows NT clients can use their native security settings 
        dialog box to view and modify the underlying UNIX permissions.</para>

        <para>Note that this ability is careful not to compromise 
        the security of the UNIX host Samba is running on, and 
        still obeys all the file permission rules that a Samba 
        administrator can set.</para>

        <note>
        <para>
        All access to Unix/Linux system file via Samba is controlled at
        the operating system file access control level. When trying to
        figure out file access problems it is vitally important to identify
        the identity of the Windows user as it is presented by Samba at
        the point of file access. This can best be determined from the
        Samba log files.
        </para>
        </note>
</sect1>

<sect1>
        <title>How to view file security on a Samba share</title>

        <para>From an NT4/2000/XP client, single-click with the right 
        mouse button on any file or directory in a Samba mounted 
        drive letter or UNC path. When the menu pops-up, click 
        on the <emphasis>Properties</emphasis> entry at the bottom of 
        the menu. This brings up the file properties dialog
        box. Click on the tab <emphasis>Security</emphasis> and you 
        will see three buttons, <emphasis>Permissions</emphasis>,       
        <emphasis>Auditing</emphasis>, and <emphasis>Ownership</emphasis>. 
        The <emphasis>Auditing</emphasis> button will cause either 
        an error message <errorname>A requested privilege is not held 
        by the client</errorname> to appear if the user is not the 
        NT Administrator, or a dialog which is intended to allow an 
        Administrator to add auditing requirements to a file if the 
        user is logged on as the NT Administrator. This dialog is 
        non-functional with a Samba share at this time, as the only 
        useful button, the <command>Add</command> button will not currently 
        allow a list of users to be seen.</para>

</sect1>

<sect1>
        <title>Viewing file ownership</title>

        <para>Clicking on the <command>"Ownership"</command> button 
        brings up a dialog box telling you who owns the given file. The 
        owner name will be of the form :</para>

        <para><command>"SERVER\user (Long name)"</command></para>

        <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of 
        the Samba server, <replaceable>user</replaceable> is the user name of 
        the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
        is the descriptive string identifying the user (normally found in the
        GECOS field of the UNIX password database). Click on the <command>Close
        </command> button to remove this dialog.</para>

        <para>If the parameter <parameter>nt acl support</parameter>
        is set to <constant>false</constant> then the file owner will 
        be shown as the NT user <command>"Everyone"</command>.</para>

        <para>The <command>Take Ownership</command> button will not allow 
        you to change the ownership of this file to yourself (clicking on 
        it will display a dialog box complaining that the user you are 
        currently logged onto the NT client cannot be found). The reason 
        for this is that changing the ownership of a file is a privileged 
        operation in UNIX, available only to the <emphasis>root</emphasis> 
        user. As clicking on this button causes NT to attempt to change 
        the ownership of a file to the current user logged into the NT 
        client this will not work with Samba at this time.</para>

        <para>There is an NT chown command that will work with Samba 
        and allow a user with Administrator privilege connected 
        to a Samba server as root to change the ownership of 
        files on both a local NTFS filesystem or remote mounted NTFS 
        or Samba drive. This is available as part of the <emphasis>Seclib
        </emphasis> NT security library written by Jeremy Allison of 
        the Samba Team, available from the main Samba ftp site.</para>

</sect1>

<sect1>
        <title>Viewing file or directory permissions</title>

        <para>The third button is the <command>"Permissions"</command> 
        button. Clicking on this brings up a dialog box that shows both 
        the permissions and the UNIX owner of the file or directory. 
        The owner is displayed in the form :</para>

        <para><command>"SERVER\user (Long name)"</command></para>

        <para>Where <replaceable>SERVER</replaceable> is the NetBIOS name of 
        the Samba server, <replaceable>user</replaceable> is the user name of 
        the UNIX user who owns the file, and <replaceable>(Long name)</replaceable>
        is the descriptive string identifying the user (normally found in the
        GECOS field of the UNIX password database).</para>

        <para>If the parameter <parameter>nt acl support</parameter>
        is set to <constant>false</constant> then the file owner will 
        be shown as the NT user <command>"Everyone"</command> and the 
        permissions will be shown as NT "Full Control".</para>


        <para>The permissions field is displayed differently for files 
        and directories, so I'll describe the way file permissions 
        are displayed first.</para>

        <sect2>
                <title>File Permissions</title>

                <para>The standard UNIX user/group/world triple and 
                the corresponding "read", "write", "execute" permissions 
                triples are mapped by Samba into a three element NT ACL 
                with the 'r', 'w', and 'x' bits mapped into the corresponding 
                NT permissions. The UNIX world permissions are mapped into 
                the global NT group <command>Everyone</command>, followed 
                by the list of permissions allowed for UNIX world. The UNIX 
                owner and group permissions are displayed as an NT 
                <command>user</command> icon and an NT <command>local 
                group</command> icon respectively followed by the list 
                of permissions allowed for the UNIX user and group.</para>

                <para>As many UNIX permission sets don't map into common 
                NT names such as <command>"read"</command>, <command>
                "change"</command> or <command>"full control"</command> then 
                usually the permissions will be prefixed by the words <command>
                "Special Access"</command> in the NT display list.</para>

                <para>But what happens if the file has no permissions allowed 
                for a particular UNIX user group or world component ? In order 
                to  allow "no permissions" to be seen and modified then Samba 
                overloads the NT <command>"Take Ownership"</command> ACL attribute 
                (which has no meaning in UNIX) and reports a component with 
                no permissions as having the NT <command>"O"</command> bit set. 
                This was chosen of course to make it look like a zero, meaning 
                zero permissions. More details on the decision behind this will 
                be given below.</para>
        </sect2>
        
        <sect2>
                <title>Directory Permissions</title>

                <para>Directories on an NT NTFS file system have two 
                different sets of permissions. The first set of permissions 
                is the ACL set on the directory itself, this is usually displayed 
                in the first set of parentheses in the normal <command>"RW"</command> 
                NT style. This first set of permissions is created by Samba in 
                exactly the same way as normal file permissions are, described 
                above, and is displayed in the same way.</para>

                <para>The second set of directory permissions has no real meaning 
                in the UNIX permissions world and represents the <command>
                "inherited"</command> permissions that any file created within 
                this directory would inherit.</para>

                <para>Samba synthesises these inherited permissions for NT by 
                returning as an NT ACL the UNIX permission mode that a new file 
                created by Samba on this share would receive.</para>
        </sect2>
</sect1>
        
<sect1>
        <title>Modifying file or directory permissions</title>

        <para>Modifying file and directory permissions is as simple 
        as changing the displayed permissions in the dialog box, and 
        clicking the <command>OK</command> button. However, there are 
        limitations that a user needs to be aware of, and also interactions 
        with the standard Samba permission masks and mapping of DOS 
        attributes that need to also be taken into account.</para>

        <para>If the parameter <parameter>nt acl support</parameter>
        is set to <constant>false</constant> then any attempt to set 
        security permissions will fail with an <command>"Access Denied"
        </command> message.</para>

        <para>The first thing to note is that the <command>"Add"</command> 
        button will not return a list of users in Samba (it will give 
        an error message of <command>"The remote procedure call failed 
        and did not execute"</command>). This means that you can only 
        manipulate the current user/group/world permissions listed in 
        the dialog box. This actually works quite well as these are the 
        only permissions that UNIX actually has.</para>

        <para>If a permission triple (either user, group, or world) 
        is removed from the list of permissions in the NT dialog box, 
        then when the <command>"OK"</command> button is pressed it will 
        be applied as "no permissions" on the UNIX side. If you then 
        view the permissions again the "no permissions" entry will appear 
        as the NT <command>"O"</command> flag, as described above. This 
        allows you to add permissions back to a file or directory once 
        you have removed them from a triple component.</para>

        <para>As UNIX supports only the "r", "w" and "x" bits of 
        an NT ACL then if other NT security attributes such as "Delete 
        access" are selected then they will be ignored when applied on 
        the Samba server.</para>

        <para>When setting permissions on a directory the second 
        set of permissions (in the second set of parentheses) is 
        by default applied to all files within that directory. If this 
        is not what you want you must uncheck the <command>"Replace 
        permissions on existing files"</command> checkbox in the NT 
        dialog before clicking <command>"OK"</command>.</para>

        <para>If you wish to remove all permissions from a 
        user/group/world  component then you may either highlight the 
        component and click the <command>"Remove"</command> button, 
        or set the component to only have the special <command>"Take
        Ownership"</command> permission (displayed as <command>"O"
        </command>) highlighted.</para>
</sect1>

<sect1>
        <title>Interaction with the standard Samba create mask 
        parameters</title>

        <para>There are four parameters 
        to control interaction with the standard Samba create mask parameters.
        These are :</para>

        <para><parameter>security mask</parameter></para>
        <para><parameter>force security mode</parameter></para>
        <para><parameter>directory security mask</parameter></para>
        <para><parameter>force directory security mode</parameter></para>

        <para>Once a user clicks <command>"OK"</command> to apply the 
        permissions Samba maps the given permissions into a user/group/world 
        r/w/x triple set, and then will check the changed permissions for a 
        file against the bits set in the <ulink url="smb.conf.5.html#SECURITYMASK"> 
        <parameter>security mask</parameter></ulink> parameter. Any bits that 
        were changed that are not set to '1' in this parameter are left alone 
        in the file permissions.</para>

        <para>Essentially, zero bits in the <parameter>security mask</parameter>
        mask may be treated as a set of bits the user is <emphasis>not</emphasis> 
        allowed to change, and one bits are those the user is allowed to change.
        </para>

        <para>If not set explicitly this parameter is set to the same value as 
        the <ulink url="smb.conf.5.html#CREATEMASK"><parameter>create mask
        </parameter></ulink> parameter. To allow a user to modify all the
        user/group/world permissions on a file, set this parameter 
        to 0777.</para>

        <para>Next Samba checks the changed permissions for a file against 
        the bits set in the <ulink url="smb.conf.5.html#FORCESECURITYMODE">
        <parameter>force security mode</parameter></ulink> parameter. Any bits 
        that were changed that correspond to bits set to '1' in this parameter 
        are forced to be set.</para>

        <para>Essentially, bits set in the <parameter>force security mode
        </parameter> parameter may be treated as a set of bits that, when 
        modifying security on a file, the user has always set to be 'on'.</para>

        <para>If not set explicitly this parameter is set to the same value 
        as the <ulink url="smb.conf.5.html#FORCECREATEMODE"><parameter>force 
        create mode</parameter></ulink> parameter.
        To allow a user to modify all the user/group/world permissions on a file
        with no restrictions set this parameter to 000.</para>

        <para>The <parameter>security mask</parameter> and <parameter>force 
        security mode</parameter> parameters are applied to the change 
        request in that order.</para>

        <para>For a directory Samba will perform the same operations as 
        described above for a file except using the parameter <parameter>
        directory security mask</parameter> instead of <parameter>security 
        mask</parameter>, and <parameter>force directory security mode
        </parameter> parameter instead of <parameter>force security mode
        </parameter>.</para>

        <para>The <parameter>directory security mask</parameter> parameter 
        by default is set to the same value as the <parameter>directory mask
        </parameter> parameter and the <parameter>force directory security 
        mode</parameter> parameter by default is set to the same value as 
        the <parameter>force directory mode</parameter> parameter. </para>

        <para>In this way Samba enforces the permission restrictions that 
        an administrator can set on a Samba share, whilst still allowing users 
        to modify the permission bits within that restriction.</para>

        <para>If you want to set up a share that allows users full control
        in modifying the permission bits on their files and directories and
        doesn't force any particular bits to be set 'on', then set the following
        parameters in the &smb.conf; file in that share specific section :</para>

        <para><parameter>security mask = 0777</parameter></para>
        <para><parameter>force security mode = 0</parameter></para>
        <para><parameter>directory security mask = 0777</parameter></para>
        <para><parameter>force directory security mode = 0</parameter></para>
</sect1>

<sect1>
        <title>Interaction with the standard Samba file attribute 
        mapping</title>

        <para>Samba maps some of the DOS attribute bits (such as "read 
        only") into the UNIX permissions of a file. This means there can 
        be a conflict between the permission bits set via the security 
        dialog and the permission bits set by the file attribute mapping.
        </para>

        <para>One way this can show up is if a file has no UNIX read access
        for the owner it will show up as "read only" in the standard 
        file attributes tabbed dialog. Unfortunately this dialog is
        the same one that contains the security info in another tab.</para>

        <para>What this can mean is that if the owner changes the permissions
        to allow themselves read access using the security dialog, clicks
        <command>"OK"</command> to get back to the standard attributes tab 
        dialog, and then clicks <command>"OK"</command> on that dialog, then 
        NT will set the file permissions back to read-only (as that is what 
        the attributes still say in the dialog). This means that after setting 
        permissions and clicking <command>"OK"</command> to get back to the 
        attributes dialog you should always hit <command>"Cancel"</command> 
        rather than <command>"OK"</command> to ensure that your changes 
        are not overridden.</para>
</sect1>

</chapter>